A monopole type antenna that is usable in a mobile communication system includes, for example, a rectangular ground plate including a feeding point, and a radiation element having one end connected to the ground plate at the feeding point. The long side of the ground plate and the radiation element collaborate with each other to operate as a dipole antenna (see, for example, Patent Document 1).
Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-282091
Therefore, the size of a ground is determined such that the long side of the ground plate or the ground and the radiation element may collaborate with each other.
Meanwhile, the number of components, the types of components, and the size of the components that are accommodated in the radio device, differ according to the purpose of the radio device. Therefore, in the radio device depending on the purpose of the radio device, it may be difficult to secure a sufficiently large ground such that the long side of the ground and the radiation element may appropriately collaborate with each other. For a monopole type antenna, when an appropriate ground is not secured, the properties of the antenna, such as the gain, may be degraded.